A car which has an internal combustion engine also comprises a pressure accumulator (also called common rail) in order to accumulate fuel and to be able to inject the fuel into one or more cylinders in a controlled manner.
A customary common rail system comprises, inter alia, the following components:                a pressure accumulator (common rail) which stores fuel under a certain pressure,        a high pressure pump which supplies fuel to the pressure accumulator,        corresponding valves which can be activated by an engine control system and which enable the pressure in the common rail to be regulated via a manipulation of the fuel inflow into the pressure accumulator or of the fuel outflow from the pressure accumulator,        a pressure sensor which measures the current pressure in the common rail (referred to below as rail pressure) and which can optionally indicate an erroneous value,        one or more injection valves (injectors) which connect the common rail to one cylinder each of the internal combustion engine and which can be opened and closed by an engine control system. In the open state, the pressure differential between the common rail and the cylinder causes a certain quantity of fuel to be injected into the cylinder, wherein said quantity of fuel depends on the opening duration of the injectors and the rail pressure,        optionally a lambda sensor,        a motor control system which, inter alia,                    1. predefines a desired value for the rail pressure for the current operating state of the engine,            2. ascertains the rail pressure P with the aid of the pressure sensor,            3. compares the desired pressure value with the actual value P and, by means of a suitable control algorithm and corresponding activation of the above-described valves, attempts to adjust the pressure measured to the corresponding desired value,            4. calculates the quantity of fuel M to be injected for the current operating state of the engine,            5. calculates, at least from the parameters M and P, an opening duration t of the injectors that is suitable for also actually realizing the desired quantity M.                        
The pressure sensor therefore has the task of measuring the current rail pressure P and of making it available to the engine control system. The latter, using this measured value, ascertains the parameters required for realizing the predefined quantity of fuel to be injected. A pressure sensor which is inaccurate (for example due to drift effects) can therefore result in inaccurate injections. In order to reduce this risk, a regular plausibility check of the value ascertained by the sensor is required.
According to legal stipulations, sensors in a car have to be subjected to a plausibility test in order to check the correct functioning of the sensors. In particular, a plausibility test of the pressure sensor in the pressure accumulator is known.
For the plausibility check of the pressure sensor, conventionally the two methods below are primarily customary:
The first method evaluates the measured value of the pressure system in states of the hydraulic system, in which the actual pressure has to be zero (for example a sufficient interval after the high pressure pump has been switched off). If, nevertheless, the pressure sensor supplies a value which is different from zero, it can be concluded that there is an error in the sensor. A disadvantage of this method is that said method is sensitive only for measuring the zero points of pressure. Errors which do not change the zero point therefore cannot be detected.
The second method is used primarily in systems having a lambda sensor (in particular petrol engines). Use is made here of the fact that, in the event of an error of measurement of the rail pressure, the injection valves are opened for a longer or shorter time than would be required in order to achieve the desired quantity of fuel to be injected. The quantity actually injected therefore does not match the provided quantity of air. As a result, lambda will differ from the target value. A disadvantage of this method (inter alia) is that the sensitivity of said method decreases with increasing pressure. Errors of measurement of the pressure sensor that result primarily at higher pressures are therefore not detectable therewith.